1. Field of the Invention
The present invention relates to a rubber composition suitable for use in a tire, a tire using the rubber composition, and a method for manufacturing a tire.
2. Related Art
An icy and snowy road surface has remarkably low friction coefficient as compared with an ordinary road surface and becomes slippery. For this reason, in a tread rubber of a tire for winter season (winter tire) such as a studless tire or a snow tire, its rubber hardness is set low using diene rubber having low glass transition temperature in order to enhance running performance on an icy and snowy road surface (hereinafter simply referred to as “on-ice performance). Furthermore, to increase friction force to an icy and snowy road surface, a method of forming a tread rubber by foamed rubber, a method of adding vegetable granules, a pulverized product of porous carbonized material, porous cellulose particles or the like to a tread rubber, and the like are proposed (for example, JP-A-2011-012110).
US2004/0019135A1 proposes adding short fibers, particles having Mohs hardness of 5 or higher, and a starch/plasticizer composite material to diene rubber in order to improve on-ice performance and wet grip performance. Furthermore, US2002/0037950A1 proposes adding short fibers, used paper and silica to diene rubber in order to improve on-ice performance.
On the other hand, although not related to on-ice performance, JP-A-2007-112834 discloses to increase rigidity of a rubber composition for a tire by adding paper fibers and a phenol type curing resin to diene rubber. Furthermore, JP-A-2005-133025 discloses to improve abrasion resistance by adding starch and cellulose to diene rubber.
Thus, in a rubber composition for a tire, use of starch is disclosed in US2004/0019135A1 and JP-A-2005-133025, and use of used paper and paper fibers is disclosed in US2002/0037950A1 and JP-A-2007-112834. However, those patent documents do not disclose to use pellets comprising a mixture of paper powder and starch and containing a given amount of water.
On the other hand, JP-A-2004-018603 discloses pellets comprising a mixture of starch, paper powder and an inorganic compound and containing water. However, this patent document relates to pellets as a raw material of a foamed body prepared using an extruder, and does not suggest that the pellets are used in a rubber composition.
As described above, the technology of using a foamed rubber in a tread rubber in order to improve on-ice performance of a tire has been known. Conventionally, an organic solvent is used as a foaming agent for the foamed rubber. However, where a rubber is foamed using an organic solvent, load applied to environment is large. Furthermore, to improve on-ice performance, there is the technology of adding hard particles having scratch effect and porous particles having removal effect of water screen on the ice. However, sufficient effect is not always obtained by only the addition of those particles, and further improvement of on-ice performance is required.